The first MC for a female dog is depending on the breed(size) and the time of maturity. For small breed dog, it can be around 5-6 months old. For large breed, it can be around 7-8 months or one year old. Do not forget the female dog only in heat during spring and fall quarters. Sometimes you won't see a small breed dog in heat until 8-10 months old because the time they are mature is not on the in heat season.(Can you understand it? Please let me know if you won't) I always recommend do not to breed your female dog in their first or second in heat. The best time to start to breed your dog is more than 1 or 1 1/2 years old(small breed) and 2 years old (large breed).
Dr. Lee